Polyimide polymers have been used for several decades as thermally and chemically resistant materials in a variety of applications. In addition, they have superior mechanical properties which make them valuable for the manufacturing of various electronic devices.
Polyimide (PI) polymers are typically prepared by chemical or thermal treatment of their precursors, polyamic acid (PAA) polymers or polyamic ester (PAE) polymers. While most PAA and PAE polymers are soluble in the polar, aprotic polymerization solvents in which they are synthesized, the resulting PI polymers formed upon imidization are usually insoluble. In many applications, since the PI polymer is not soluble, the PAA or PAE polymer is coated onto a substrate and cured to temperatures in excess of 350° C. to form the PI polymer. In recent advanced applications, coating of a soluble PI polymer is more preferable than coating of its PAA or PAE precursors because of lower required curing temperature and lower film thickness loss due to curing, which results in less stress on the substrate.
In those cases where the PI polymer remains soluble, isolation and purification are accomplished by addition of the polymerization solution to a large amount of a non-solvent. See for example U.S. Pat. Nos. 3,856,752, 4,026,876, 5,252,534, 5,478,915, US20040265731 and US20040235992 which are incorporated by reference. Typical non-solvents are water, low boiling alcohols, such as methanol and 2-propanol, or hydrocarbon solvents such as hexane or toluene. The precipitated polymer is then filtered, washed with an additional large amount of non-solvent and dried under vacuum at elevated temperature. In most cases, in order to obtain a material of sufficient purity, the polymer must be re-dissolved in a solvent and precipitated a second time into a non-solvent. Presence of impurities in PI polymers results in compositions with inferior properties such as mechanical, electrical or chemical resistance. In addition, presence of minute amount of undesired polar aprotic polymerization solvents in the final polymer or compositions from those polymers is undesirable from environmental, safety and health perspective. These methods are also used to isolate PAA and PAE polymers. In this way, conventional processes used to produce PI, PAA and PAE polymers will often generate from 100 kilograms to 500 kilograms of waste for every 1 kilogram of polymer produced. Additionally, using conventional methods, it is exceedingly difficult to reduce the amount of residual solvent to a desired level (e.g., less than 1 wt %) due at least in part to the strong association of the undesired polar aprotic polymerization solvent with the PI, PAA and PAE polymers. Moreover, the excessive amount of polymer handling resulting from multiple precipitations, filtrations and drying steps can further compromise the purity of the polymers from contamination with species such as trace metals.